The invention relates to the use of liquid nitrogen to enhance the operation of a closed loop refrigeration system for industrial plants.
Many industrial processes require refrigeration systems. For example, the recovery of olefins from gas mixtures is an economically important but highly energy intensive petrochemical process. In general, the gas mixtures are produced by hydrocarbon pyrolysis in the presence of steam (via thermal cracking, fluid catalytic cracking or fluid coking processes). Thereafter, Cryogenic separation methods are commonly used to recover the olefins, such methods requiring large amounts of refrigeration at low temperatures.
A more specific example is an ethylene production plant. Refrigeration is required to separate desired products from the cracking heater effluent. The refrigeration may be provided by water cooling, closed cycle propylene and ethylene systems, or work expansion of pressurized light gases from the separation process.
Also, in plants of this type, gaseous nitrogen is required for numerous uses within the plant. It is typical for the nitrogen to be delivered to the plant as a cryogenic liquid. The liquid nitrogen must be vaporized and heated in order to provide nitrogen gas at usable temperatures and pressures. Typically, this is done using air at ambient condition to vaporize and heat the nitrogen. Nitrogen vaporizes below −14° C. The vaporizing and heating can be energy use intensive. For example, to heat nitrogen to 35° C. ambient conditions requires about 83 calories per gram of nitrogen. A plant needing 100 kilowatts of refrigeration will generally need 1,000 kg/hr of nitrogen. Therefore the energy required for heating the nitrogen is in the range of 83 million calories per hour, e.g. a considerable amount.
FIG. 1 shows a refrigeration system as known in the prior art. In the system shown in FIG. 1, the 2nd Stage Refrigerant Compressor discharge is condensed in the Refrigerant Condenser before entering the Refrigerant Accumulator. Refrigerant liquid is flashed to a lower pressure and then partially vaporized in the 2nd Stage Refrigerant User. The refrigerant than enters the 2nd Stage Suction Drum where liquid is removed and then sent to the 1st Stage Refrigerant User, where the refrigerant is flashed to a lower pressure and completely vaporized. The vapor form the 2nd Stage Suction Drum is returned to the 2nd Stage Refrigerant Compressor. The vapor from the 1st Stage Refrigerant User is processed in the 1st Stage Suction Drum to remove any entrained liquids and then sent on to the 1st Stage Refrigerant Compressor.
For use in an ethylene plant, a typical closed loop refrigeration system is shown in FIG. 1. Inherent limitations of the refrigeration system often limit the production capacity of the plant, which in the industry is referred to as a “bottleneck”. To relieve this bottleneck, the addition of refrigeration capacity may be necessary, in which case, expensive modifications or replacement of compressors, heat exchangers, drums and the like may be required. Even if the refrigeration system is not a plant bottleneck, addition of cooling duty to the refrigeration system and removal of inerts via recovery of refrigerant significantly reduces refrigerant compressor power demand and therefore significantly reduce energy consumption and associated operating expenses.
There remains a need in the art for improvements to refrigeration systems for use in industrial plants, such as petrochemical plants.